Stefan Bader

Characterization of reversed c-axis AlN thin films

Background: It is desired to grow AlN in a reversed c-axis configuration to fabricate R-FBARS (Reversed c-axis Film Bulk Acoustic Resonator) with reactively sputtered, thin film Aluminum Nitride (AlN). Previous methods of growing reversed c-axis AlN, result in films with low electro-acoustic coupling constant, low Q, or inability to withstand the Avago Technologies FBAR release process. Conribution/Methods: An AMS Inc. deposition tool, modified to allow independent control and unique processes, was used to deposit AlN on Al, Mo or W bottom electrodes used in this study. AlN films of thickness 1.2 microns were deposited over patterned bottom electrodes, with additional processing used to reverse the c-axis of the AlN. A top electrode was deposited, patterned, and either processing stopped at the transducer point, or the R-FBAR was released from the silicon wafer with HF acid. Results: The Avago Technologies Acoustic Imaging Microscope interferometer (AIM) is used in point mode to determine the AlN c-axis orientation. The transducer or R-FBAR is driven with a 40 kHz sine wave, and the phase of the top surface motion is observed. Both the orientation and the piezoelectric coefficient were determined. To evaluate the material constants of the AlN for the R-FBAR structure, the input RF reflection coefficient vs frequency is measured. From a one-dimensional Mason model for the R-FBAR stack, the AlN material parameters — coupling constant kt2, resonant frequency, velocity, and attenuation, were determined by varying them in the model, to backfit the measured data. R-FBAR resonators with reversed c-axis orientation, termed type CN (“Compression Negative”), as well as FBARS with normal c-axis orientation termed Type CP, (“Compression Positive”), were fabricated. A strong piezocoupling constant was observed, depending on the deposition parameters used. The voltage shift (“Voltco”) coefficient of the resonator resonant frequencies, fS or fP, was observed, compared to the interoferometric observations, and found to be a reliable indicator of c-axis polarity. For the FBARS and R-FBARS reported here, for the Type CP films, a Voltco of +40 kHz/Volt was observed, and for Type CN films, −30 kHz/Volt was observed. Low frequency RBARS and high frequency RSBARS stacked resonator structures were fabricated, and preliminary results are given.